In a capacitor discharge type fastener welding device an arc is produced by the rapid discharge of stored electrical energy in a capacitor to a fastener to be welded. The stored electrical energy vaporizes a small projection at the end of a fastener and creates an arc gap. Pressure may be applied to the fastener and the workpiece to be welded following the electrical discharge plunging the fastener into the workpiece to form a weld.
Known prior art capacitor discharge welders may include a transformer and other resistors within a circuit to control the charging of a capacitor in a capacitor discharge welding system. Such prior art designs include a large number of components including large or heavy components such as a transformer leading to an overall increase in the cost and weight and heat, and loss of power conversion efficiency of such designs. The transformer serves several functions in a CD welder: one function is safety isolation between the input and output at a threshold required by international standard; and another function is to transform voltage level. If the input is 115 VAC, it is desirable to use a transformer turns ratio to step up (or boost) voltage for charging the capacitor. If the input is 230 VAC, it is desirable use a transformer turns ratio to step down (or buck) voltage for charging the capacitor. In addition, the prior art with transformer also requires the transformer to be “re-tapped” to accommodate different input voltages to obtain the desired turns ratio. It is the goal of this patent to eliminate the transformer while preserving these functions and accepting both input voltages automatically without re-configuring or re-wiring the circuit.
Another prior art capacitor discharge welder design includes silicon controlled rectifiers (SCRs) that are used to connect and disconnect the capacitor from an input voltage source directly without a transformer. Isolation to the input voltage source normally provided by a transformer is performed by the SCRs as part of the rectifier. However, such prior art design includes limitations in that silicon controlled rectifiers may not be turned off immediately when they are in a conducting state. For example, a silicon controlled rectifier generally cannot be turned off until the current flow through the silicon controlled rectifier falls below a specific minimum threshold level such that the current input is crossing zero. In such designs, the SCRs do not provide disconnection from an input voltage source immediately. For example, a delay can be as long as 8.3 milliseconds when using a 60 Hertz line frequency input power.
Additionally, welding power supply designs must meet international standards such as TEC 60974-1 which requires protection against electric shock and must withstand overvoltage conditions. The peak voltage required is 2950V at 115 VAC input and 5067V at 230 VAC input. A typical SCR has a non-repetitive/repetitive reverse/forward blocking voltage of 200V-2400V. In order to comply with IEC 60974-1, prior art using SCR to isolate input power must use specialty SCR with high blocking voltage rating at high cost and low availability.
Additionally, prior art with SCR controlled capacitor charging circuit without transformer cannot achieve a capacitor DC voltage higher than the rectified input AC power. In welding applications, it is desirable to regulate to a higher voltage and the ability to weld at the same capacitor voltage when input voltage is below the nominal by 10%. At 115 VAC input, prior art without transformer is not capable of welding at voltage higher than 163V. Aluminum fasteners are soft and have high conductivity, which requires about 190V. Copper, brass, copper-zinc alloys and galvanized steel all require higher voltage up to 200V.
There is therefore a need in the art for an improved capacitor discharge welding circuit that includes a low cost and reliable system that allows for immediate disconnect between a capacitor and an input voltage source. Further, there is a need in the art for a capacitor discharge welding circuit that is capable of handling high break over voltages. Additionally, there is a need in the art for a capacitor discharge welding circuit that has the ability to regulate and adjust the voltage based on various application needs.